Optical conductors are commonly employed in the field of telecommunications. Information in light form is generally conveyed at a wavelength of about 1300 nm or of about 1550 nm via silica-based optical fibers. Each fiber is protected by layers of polymer material, and the protective layers are usually covered in another polymer that is pigmented.
A set of optical fibers can be assembled together to form a tape. Under such circumstances, the tape material is also a polymer.
The optical fibers, whether individual or in a tape, are placed inside a tube made of metal or of plastics material.
It is known that optical fibers must not be exposed to hydrogen since that gas spoils their transmission properties. The extent of spoilage increases with increasing hydrogen partial pressure to which the fiber is subjected.
For example, when the partial pressure exerted by hydrogen is one atmosphere, then the attenuation of the fiber increases by about 6 dB/km at a wavelength of 1240 nm. At 1240 nm, this attenuation is substantially proportional to the partial pressure of hydrogen.
It may be observed at this point that although the fibers are used at a wavelength of 1300 nm or of 1550 nm, variation in attenuation is often measured at a wavelength of 1240 nm, since that is the wavelength at which fiber presents its greatest sensitivity to hydrogen.
Hydrogen can be produced, in particular, by decomposition of the polymers constituting the fiber coatings or the material for assembling the fibers together in the form of a tape. Hydrogen can also come from the decomposition of the filler material that is generally provided in a tube in order to hold the fibers inside the tube and impede ingress of moisture in the event of the tube being broken or damaged.
The decomposition mentioned above occurs naturally because of aging.
When the tube is made of plastics material, then the porosity of the material allows the hydrogen to escape. However, when the tube is made of metal, the hydrogen remains confined inside the tube and the fibers are therefore progressively degraded.
Until now, two solutions have been proposed for remedying this drawback.
The first solution consists in providing a material that absorbs hydrogen. That solution is expensive.
The second solution consists in avoiding the use of a filler material in the tube. Under such circumstances, the cable is not protected against ingress of water into the tube and the fibers can also be subjected to vibration. In addition, the fibers can move inside the tube and that can lead to damage.